Method of and apparatus for encoding signals, and method of and apparatus for decoding the encoded signals

ABSTRACT

A method of and an apparatus for encoding and decoding using transformation bases of a yet higher efficiency. In a method for encoding an object signal in compliance with a transformation rule, a signal correlating to the object signal is obtained, and a transformation base that forms the transformation rule is derived based on a characteristic of the obtained reference signal. The object signal is transformed and encoded in compliance with the transformation rule based on the derived transformation base. Accordingly, the object signal is transformed in compliance with the transformation rule based on the transformation base derived from the characteristic of the reference signal. Since the reference signal is correlated to the object signal, the transformation base derived from the characteristic matches the feature of the object signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/297,492, filed Dec. 9, 2002, which is a National Stage Application ofPCT Application No. PCT/JP02/03499, filed Apr. 8, 2002, and claimspriority to Japanese Application No. 2001-110664, filed Apr. 9, 2001.The entire contents of U.S. patent application Ser. No. 10/297,492 areincorporated herein by reference in their entirety.

The present invention generally relates to a method of and an apparatusfor encoding and decoding a series of signals, and more particularly, toa method of and an apparatus for encoding and decoding a series ofsignals using a transformation basis as DCT.

BACKGROUND OF THE INVENTION

Conventionally, an image encoding apparatus and an image decodingapparatus based on MPEG-1 encoding method are disclosed in Le Gall, D.“MPEG: A Video Compression Standard for Multimedia Applications” (Trans.ACM, 1991, April). The image encoding apparatus is constructed as showedin FIG. 1, and the image decoding system is constructed as showed inFIG. 2.

The image encoding apparatus showed in FIG. 1 reduces redundancy in thetime directions by motion compensating inter-frame prediction, andfurther reduces redundancy remaining in the spatial directions by DCT(Discrete Cosine Transform) to compress an image signal. FIG. 3 showsmotion compensating inter-frame prediction; FIG. 4 shows block matchingmethod frequently used to detect a motion vector; FIG. 5 shows theconcept of DCT; and FIG. 6A shows the principle of encoding DCTcoefficients. The operations of the image encoding apparatus and theimage decoding apparatus showed in FIGS. 1 and 2, respectively, will bedescribed by reference to these drawings.

An input image signal 1 is a time series of framed images and,hereinafter, refers to a signal by a framed image. A framed image to beencoded will be called a current frame as showed in FIG. 3. The currentframe is divided into 16 pixels×16 lines square regions (hereinafterreferred to as a “macro block”), for example, and dealt with as follows.

The macro block data (current macro block) of the current frame are sentto motion detection unit 2 to detect a motion vector 5. A patternsimilar to the current macro block is selected from patterns in apredetermined search region of encoded framed images 4 (hereinaftercalled partially decoded images) stored in a frame memory 3, and themotion vector 5 is generated based on the spatial distance between theselected pattern and the current macro block.

The above partially decoded image is not limited to frames in the past.It is possible to use frames in the future by encoding them in advanceand storing them in a frame memory. The use of the future framesincreases time required for processing since the order of encoding needsto be switched. The use of the future frames, however, further reducesredundancy in the time directions effectively. Generally, in the case ofMPEG-1, the following encoding types are selectively available:bi-directional prediction using both a past frame and a future frame(B-frame prediction); prior-directional prediction using only a priorframe (P-frame prediction); and I-frame that performs encoding withoutprediction. In FIG. 3 showing the case of P-frame prediction, thepartially decoded image is indicated as a prior frame.

The motion vector 5 is represented by a two dimensional translation. Themotion vector 5 is usually detected by block matching method showed inFIG. 4. A search range centered at the spatial position of the currentmacro block is provided, and motion is searched in the motion searchrange. A motion prediction datum is defined as a block that minimizesthe sum of squared differences or the sum of absolute differencesselected from the image data in the motion search range of the priorframe. The motion vector 5 is determined as the quantity of positionalchange between the current macro block and the motion prediction data. Amotion prediction datum is obtained for each macro block of the currentframe. The motion prediction data represented as a frame imagecorresponds to a motion prediction frame of FIG. 3. For the motioncompensation inter-frame prediction, a difference between the motionprediction frame and the current frame is obtained, and the remaindersignal (hereinafter referred to as prediction remainder signal 8) isencoded by DCT encoding method as showed in FIG. 3.

Specifically, a motion compensation unit 7 identifies the motionprediction datum of each macro block (hereinafter referred to asprediction image). That is, this motion compensation unit 7 generates aprediction image 6 from the partially decoded image 4 stored in theframe memory 3 using the motion vector 5.

The prediction remainder signal 8 is converted into a DCT coefficientdatum by a DCT unit 9. As showed in FIG. 5, DCT converts a spatial pixelvector into a combination of normal orthogonal bases each representing afixed frequency element. A block of 8×8 pixels (hereinafter referred toas a DCT block) is usually employed as a spatial pixel vector. Since DCTis a separation type conversion, each eight dimensional horizontal rowvector of a DCT block is separately converted, and each eightdimensional vertical column vector of a DCT block is separatelyconverted. DCT localizes power concentration ratio in a DCT block usingthe inter-pixel correlation existing in the spatial region. The higherthe power concentration ratio is, the more efficient the conversion is.In the case of a natural image signal, the performance of DCT is as highas that of KL transformation that is the optimum conversion. Especially,the electric power of a natural image is mainly concentrated in a lowfrequency range and little distributed to the high frequency range.Accordingly, as showed in FIG. 6B, the quantization coefficients arescanned in the DCT block in a direction from a low frequency to a highfrequency. Since the scanned data includes many zero runs, the totalencoding efficiency including the effect of entropy encoding isimproved.

A quantization unit 11 quantizes the DCT coefficients 10. The quantizedcoefficients 12 are scanned by a variable length encoding unit 13 andconverted into a run-length code that is multiplexed on a compressedstream 14 and transmitted. In addition, the motion vector 5 detected bythe motion detection unit 2 is multiplexed on the compressed stream 14by a macro block and transmitted for the generation by a image decodingapparatus of the same prediction image as that generated by the imageencoding apparatus.

A quantized coefficient 12 is partially decoded via an inversequantization unit 15 and an inverse DCT unit 16. The result is added tothe predicted image 6 to generate a decoded image 17 that is the same asa decoded image data generated by the image decoding apparatus. Thedecoded image 17 is stored in the frame memory 3 as the partiallydecoded image to be used for the prediction of the next frame.

The operation of an image decoding apparatus showed in FIG. 2 will bedescribed below.

This image decoding apparatus, after receiving a compressed stream 14,detects a sync word indicating the top of each frame by a variablelength decoding unit 18 and restores the motion vector 5 and thequantized DCT coefficient 12 by a macro block. The motion vector 5 istransferred to the motion compensation unit 7 that extracts a portion ofimage stored in a frame memory 19 (that is used in the same manner asthe frame memory 3) that moved for the motion vector 5 as the predictionimage 6. The quantized DCT coefficient 12 is restored through a inversequantization unit 15 and a inverse DCT unit 16, and then, added to thepredicted image 6 to make the final decoded image 17. The decoded image17 is output to a display device at a predetermined timing to reproducethe image.

Encoding algorisms such as MPEG motion picture encoding that utilize acorrelation of a signal that has already been decoded (hereinafterreferred to as a reference image or a prediction image) are widelyemployed as described in connection with the conventional exampledescribed above. DCT is frequently used as the transformation basebecause of the reasons described above. DCT is effective for encodingsignal waveforms the prior probability distribution of which is unknown.However, media signals such as an audio signal and an image signal aregenerally unsteady and spatially and temporally biased. Accordingly, inthe case of the fixed transformation base described above in connectionwith the conventional example, the number of the bases (the number ofcoefficients) cannot be reduced, which poses a limit on the compression.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide a methodof and an apparatus for encoding and decoding using transformation basesat even higher efficiency.

To achieve the above objects, a method of encoding an object signal incompliance with a transformation rule, as described in claim 1, includesa first step of obtaining a reference signal correlating to said objectsignal, a second step of deriving a transformation base that forms saidtransformation rule based on a characteristic of the obtained referencesignal, and a third step of encoding said object signal in compliancewith the transformation rule formed by the derived transformation base.

The method of encoding a signal as described above, the object signal istransformed in compliance with the transformation rule based on thetransform base derived based on the characteristic of the referencesignal. Since this reference signal correlates to the object signal, thederived transformation base matches the feature of the object signal.

The above object signal is, for example, an image signal indicatinginformation in connection with an image, a media signal such as an audiosignal, and any other signal.

When the image signal is input as the object signal, it is possible touse predicted remainder signal that is obtained by motion compensationprediction method from the input original image signal as the objectsignal. Additionally, in the case the image signal is used as the objectsignal, it is possible to use predicted image signal that is obtained bymotion compensation prediction method from the input image signal as thereference signal.

From the standpoint that the decoding side, even if information aboutthe transformation base is not transmitted from the encoding side to thedecoding side, can reproduce the transformation base that is used forencoding by the encoding side, as described in claim 2, said referencesignal is identical to a signal that is to be obtained when the objectsignal encoded by the method is decoded.

The above transformation base can be generated, as described in claim 6,based on the characteristic of the reference signal. In addition, thetransformation base, as described in claim 11, can be selected based onthe characteristic of the reference signal from a predeterminedplurality of transformation bases.

In the case where a transformation base to be used is selected from thepredetermined plurality of transformation bases, the operation of thedecoding side is made easy if the decoding side is provided with thesame plurality of transformation bases. In this case, as described inclaim 15, it is possible to construct so that information to identifythe selected transformation base is encoded with the object signal. Bytransmitting the information to identify this encoded transformationbase, the decoding side can identify a transformation base used by thedecoding side from the plurality of transformation bases based on theinformation to identify the transformation base.

From the standpoint that the decoding side can easily obtain thetransformation base used by the encoding side, the present invention, asdescribed in claim 17, can encode the transformation base derived asdescribed above with the object signal. Since this transformation baseis encoded and transmitted to the decoding side, the decoding side caneasily obtain the transformation base used by the encoding side.

In the case where an appropriate transformation is impossible with onlythe predetermined plurality of transformation bases, it is effective, asdescribed in claim 18, to generate the transformation base based on thecharacteristic of the above reference signal, and select atransformation base to be used based on the characteristic of thereference signal from the predetermined plurality of transformationbases and the generated transformation base.

In order to avoid such a situation that an appropriate transformationbase is not included in the plurality of transformation bases as much aspossible, the present invention, as described in claim 19, in the casewhere the generated transformation base is selected as a transformationbase to be used, can add the generated transformation base to theplurality of transformation bases.

In order to avoid unnecessary increase in the number of transformationbases including in the plurality of transformation bases, the presentinvention can be constructed, as described in claim 20, so that atransformation base determined based on said characteristic of saidreference signal and a mutual relationship with another one of saidplurality of transformation bases is deleted from said plurality oftransformation bases.

In order to use more appropriate transformation base, the presentinvention is constructed, as described in claim 23, so that one of saidplurality of transformation bases is selected based on saidcharacteristic of said reference signal, and said object signal isencoded using the selected one of said plurality of transformation basesand the generated transformation base, and either the selected one ofsaid plurality of transformation bases or the generated transformationbase is selected based on results of encoding.

In order to transform the object signal by a pattern matching methodsuch as so-called “Matching Pursuits”, the present invention isconstructed, as described in claim 29, so that a partial signal waveformof said object signal is identified, and said partial signal waveform isconverted into similarity information indicating a similarity betweensaid partial signal waveform and a waveform vector making atransformation vector, information to identify said waveform vector,said similarity information, and a position of said partial signalwaveform in said object signal are encoded, and in said second step, awaveform vector that makes a transformation base is generated based on acharacteristic of a partial signal of said reference signalcorresponding to said partial signal waveform of said object signal.

To solve the above problems, an apparatus as described in claim 33, forencoding an object signal in compliance with a transformation rule,includes a first unit that obtains a reference signal correlating tosaid object signal, a second unit that derives a transformation basethat forms said transformation rule based on a characteristic of theobtained reference signal, and a third unit that encodes said objectsignal in compliance with the transformation rule formed by the derivedtransformation base.

In addition, to achieve the above object, a method, as described inclaim 62, of decoding an encoded signal and transforming the decodedsignal in compliance with a transformation rule to reproduce an imagesignal, includes a first step of deriving a transformation base thatforms said transformation rule based on the decoded signal, and a secondstep of transforming the decoded signal in compliance with saidtransformation rule based on the derived transformation base toreproduce said image signal.

From the standpoint that the decoding side can generate thetransformation base, the present invention is constructed, as describedin claim 63, so that, in said first step, a signal correlated to thedecoded signal is obtained as a reference signal, and saidtransformation base is generated based on a characteristic of theobtained reference signal.

In order to make the operation of the decoding side to obtain thetransformation base, the present invention is constructed, as describedin claim 72, so that, in said first step, a transformation base to beused in said second step is selected from a plurality of predeterminedtransformation bases based on a characteristic of said reference signal.

In order to make the operation of the decoding side easier, the presentinvention is constructed, as described in claim 78, so that, in saidfirst step, a transformation base obtained by decoding said encodedsignal is obtained as a transformation base to be used in said secondstep.

Additionally, the present invention can be constructed, as described inclaim 79, so that, in said first step, in the case where a firsttransformation base that is not included in said plurality oftransformation bases is included in a signal obtained by decoding saidencoded signal, said first transformation base is obtained as atransformation base to be used in said second step, and said secondtransformation base is added to said plurality of transformation bases.

In the method of decoding signal, in order to avoid the unnecessaryincrease in the number of the transformation base, the present inventioncan be constructed, as described in claim 80, so that, in the case whereinformation to identify said second transformation base in saidplurality of transformation bases is included in a signal obtained bydecoding said encoded signal, said second transformation base is deletedfrom said plurality of transformation bases.

In order to replace the above second transformation base with the abovefirst transformation base, the present invention can be constructed, asdescribed in claim 81, so that said first transformation base isidentified by information that identifies said second transformationbase in said plurality of transformation bases.

In order to decode the signal that is encoded by the pattern matchingmethod such as so-called “Matching Pursuits”, the present invention canbe constructed, as claimed in claim 83, in the case where informationindicating that a waveform vector that makes a transformation basegenerated based on a characteristic of a partial signal of apredetermined reference signal has been used when an object signal isencoded, similarity information indicating a similarity between saidwaveform vector and said partial signal waveform of said object signal,and a position of said partial signal waveform in said object signal areincluded in a signal that is obtained by decoding said encoded signal,in said first step, a waveform vector making a transformation base,which is available from said encoded signal, is generated based on acharacteristic of said partial signal of said reference signalcorresponding to a predetermined reference signal used when said signalis encoded, in said second step, said partial signal waveform at theposition in said object signal is reproduced by transforming saidsimilarity information in compliance with said transformation rule basedon the generated waveform vector.

Furthermore, in order to achieve the above object, an apparatus, asdescribed in claim 85, for decoding an encoded signal and transformingthe decoded signal in compliance with a transformation rule to reproducean image signal, includes a first unit that derives a transformationbase that forms said transformation rule based on the decoded signal,and a second unit that transforms the decoded signal in compliance withsaid transformation rule based on the derived transformation base toreproduce said image signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a conventional image encodingapparatus for encoding using DCT technique;

FIG. 2 is a block diagram showing a conventional image decodingapparatus for decoding using DCT technique;

FIG. 3 is a schematic diagram for explaining the mechanism of motioncompensation inter-frame prediction;

FIG. 4 is a schematic diagram for explaining block matching process thatis used to detect a motion vector;

FIG. 5 is a schematic diagram for explaining the concept of DCT;

FIGS. 6A and 6B are schematic diagrams for explaining the principle ofencoding of the DCT coefficients;

FIG. 7 is a schematic diagram for explaining the principle of encodingaccording to an embodiment of the present invention;

FIG. 8 is a schematic diagram showing an image encoding apparatusaccording to the first embodiment of the present invention;

FIG. 9 is a schematic diagram showing an image decoding apparatusaccording to the first embodiment of the present invention;

FIG. 10 is a schematic diagram showing the brightness distribution in aregion of a predicted image in which orthogonal transformation isapplied;

FIG. 11 is a schematic diagram showing an image encoding apparatusaccording to the second embodiment of the present invention;

FIG. 12 is a schematic diagram showing an image decoding apparatusaccording to the second embodiment of the present invention;

FIG. 13A is a schematic diagram showing formulae of DCT transformation;

FIGS. 13B and 13C are schematic diagrams showing DCT transformationbases;

FIGS. 14A and 14B are schematic diagrams showing transformation bases(No. 1);

FIGS. 15A and 15B are schematic diagrams showing transformation bases(No. 2);

FIGS. 16A and 16B are schematic diagrams showing transformation bases(No. 3);

FIGS. 17A and 17B are schematic diagrams showing transformation bases(No. 4);

FIGS. 18A and 18B are schematic diagrams showing transformation bases(No. 5);

FIGS. 19A and 19B are schematic diagrams showing transformation bases(No. 6);

FIG. 20 is a schematic diagram showing a variation of the image encodingapparatus according to the second embodiment of the present invention;

FIG. 21 is a schematic diagram showing a variation of the image decodingapparatus according to the second embodiment of the present invention;

FIG. 22 is a schematic diagram showing an image encoding apparatusaccording to the third embodiment of the present invention;

FIG. 23 is a schematic diagram showing an image decoding apparatusaccording to the third embodiment of the present invention;

FIG. 24 is a schematic diagram showing an image encoding apparatusaccording to the fourth embodiment of the present invention;

FIG. 25 is a schematic diagram showing an image decoding apparatusaccording to the fourth embodiment of the present invention;

FIG. 26 is a schematic diagram showing an image encoding apparatusaccording to the fifth embodiment of the present invention;

FIG. 27 is a schematic diagram showing an image decoding apparatusaccording to the fifth embodiment of the present invention;

FIG. 28 is a schematic diagram showing an image encoding apparatusaccording to the sixth embodiment of the present invention; and

FIG. 29 is a schematic diagram showing an image decoding apparatusaccording to the sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A description on embodiments of the present invention will be givenbelow by reference to the drawings.

If transformation bases can be modified depending on the pattern ofimage, and transformation bases that fit the local signal distributionof the image are used, the number of coefficients to be encoded isreduced, and the efficiency of the encoding can be improved. The imagedecoding apparatus can use a reference image to modify thetransformation bases because the reference image does not need to betransmitted as additional information from the image encoding apparatusto the image decoding apparatus, and the reference image reflects thepattern of signal to be encoded (FIG. 7).

As showed in FIG. 7, a waveform pattern of the original image or thereference image, in the case of the boundary of an object, for example,to which the motion model does not fit, remains in the predictionremainder signal generated by motion compensation inter-frameprediction. Especially, an electric power often concentrates to an edgeportion (the outline of a car showed in FIG. 7, for example). DCTrequires many non-zero coefficients to express such a pattern since thetransformation bases of DCT are fixed. Accordingly, as showed in FIG. 7,it is necessary to modify the fixed transformation bases of DCT based onthe pattern of a reference image to generate new transformation bases.The new transformation bases are generated so that, in the case wherethe reference pattern includes a steep step edge, a transformation basethat can best represent the step edge on behalf of a DC element is setas a principal axis. Because these newly generated transformation basesare used instead of the fixed transformation bases of DCT, the principleaxis is set in accordance with the local frequency distribution of thesignal instead of a DC element of DCT, and the concentration ratio ofthe electric power increases.

As described above, an image encoding apparatus and an image decodingapparatus according to an embodiment of the present invention areconstructively provided with a means for modifying the transformationbases so that the concentration ratio of the electric power of eachsignal to be encoded is increased using the correlation between thesignal to be encoded (predicted remainder signal) and the referencesignal that well reflects the pattern of the signal to be encoded.Accordingly, the signal is represented by fewer coefficients, whichresults in a higher compression ratio.

The first embodiment of the present invention will be described asbelow.

An image encoding apparatus according to this embodiment is constructedas showed in FIG. 8, for example, and an image decoding apparatusaccording to this invention is constructed as showed in FIG. 9, forexample. In the present embodiment, redundancy in the temporal directionis reduced by motion compensation inter-frame prediction. The DCTtransformation bases are modified so that the waveform pattern ofpredicted image of the macro block obtained by the motion compensationprediction is captured. The signal is encoded and compressed by themodified transformation bases. The modification of the transformationbases for each pattern of the predicted image requires additionaloperations. The image encoding apparatus, however, does not need totransmit additional information in connection with the modification ofthe transformation bases to the image decoding apparatus since theoperation is performed using the predicted image data that are sharedwith the image decoding apparatus.

In the image encoding apparatus showed in FIG. 8, the procedure ofmotion compensation inter-frame prediction is identical to that of theconventional method. FIG. 3 shows the procedure of motion compensationinter-frame prediction, and FIG. 4 shows block matching processing thatis used to detect the motion vector. As showed in FIGS. 6A and 6B, theprocedure of quantizing coefficients that are obtained by transformationusing the modified orthogonal bases and encoding the quantizedcoefficients into entropy codes is identical to that of the conventionalexample. The image encoding apparatus and the image decoding apparatuswill be described below by reference to these drawings.

In FIG. 8, the input image signal 101 is a signal corresponding to aframed image in a time series of framed images (The framed image to beencoded corresponds to the current frame of FIG. 3). Each macro block ofthe current frame is encoded by the following procedure. The currentmacro block is transferred to the motion detection unit 102 that detectsmotion vector 105. A motion compensation unit 107, using the motionvector 105, extracts a predicted image 106 of each macro block from thepartially decoded image 117 stored in the frame memory 103.

A predicted remainder signal 108 is obtained as the difference betweenthe current macro block and the predicted image 106. This predictedremainder signal 108 is converted into the orthogonal transformationcoefficient data 110 by the adaptive transformation unit 109. Thetransformation bases 119 used by the adaptive transformation unit 109are generated by a transformation base operation unit 118 depending onthe pattern of the predicted image 106. The generated transformationbases 119 are transferred to the adaptive transformation unit 109 andused for the orthogonal transformation. The operation of thetransformation base operation unit 118 will be described later.

The orthogonal transformation coefficient data 110 obtained by theadaptive transformation unit 109 are quantized by the quantization unit111, encoded into run-length codes by scanning by the variable lengthencoding unit 113, and multiplexed in the compressed stream 114 fortransmission. The motion vector 105 of each macro block is multiplexedin the compressed stream 114 for transmission. The quantizedcoefficients 112 are partially decoded by an inverse quantization unit115 and a inverse adaptive transformation unit 116. The same decodedimage 117 as that of the image decoding apparatus is generated by addingthe partially decoded image to the predicted image 106. The decodedimage 117 is stored in the frame memory 103 as a partially decoded imagethat is used for the prediction of the next frame.

The transformation base operation unit 118 operates as follows.

The transformation base operation unit 118 divides the input predictedimage 106 into regions (N X N pixel blocks, where N=4, 8, and so forth)to which the orthogonal transformation is applied, obtain thetransformation bases 119 for each region, and output the transformationbases 119 to the adaptive transformation unit 109. As showed in FIG. 10,average brightness distributions x_(H) and x_(V) in the horizontal andvertical directions are obtained for each region of the predicted image106 to which the orthogonal transformation is applied. Waveform patternsthat reflect the principal components of each region in the horizontaland vertical directions are obtained. FIG. 10 shows an example of imagepattern in the case of N=4 in which a steep edge exists in thehorizontal directions but the image pattern is flat in the verticaldirections. The bases of DCT are modified so that only thetransformation coefficients of the principal axis (the first row vectorof the transformation matrix, and direct current components in the caseof DCT) and its neighbors have a considerable amount, and the averagebrightness distribution vectors x_(H) and x_(V) match the base of theprincipal axis. Particularly, the horizontal and vertical DCTtransformation bases are replaced by weighted normalized averagebrightness distribution vectors, and correlation matrices C_(H) andC_(V) are obtained. Eigenvectors φ_(H,0)-φ_(H,N-1) and φ_(V,0)-φ_(V,N-1)of the correlation matrices become the new transformation bases 119. Thetransformation bases 119 form normalized orthogonal bases.

Accordingly, the patterns of the average brightness distribution of thepredicted image 106 in the horizontal directions and the verticaldirections are reflected in the principal axis of the orthogonaltransformation. In the case where there is a similarity in patternbetween the predicted image showed in FIG. 7 and the image to be encoded(the predicted remainder signal), the concentration ratio of theorthogonal transformation coefficients of the image is increased. As analternative method of implementation, it is possible to prepare sometemplates of waveform patterns that may appear as an average brightnessdistribution vector and select a waveform pattern that maximizes theinner product with the average brightness distribution vector.

In addition, the inverse adaptive transformation unit 116 inverselyconverts the transformation coefficients into a signal in the imagespace using the transpose of the transformation bases 119.

In the image decoding apparatus showed in FIG. 9, a variable lengthdecoding unit 120 detects a sync word indicating the top of each framein the received compressed stream 114 and restores the motion vector 105and the quantized orthogonal transformation coefficients 121 of eachmacro block. The motion vector 105 is transferred to the motioncompensation unit 107. The motion compensation unit 107, in the samemanner as it operates in the image encoding apparatus, obtains a partialimage stored in the frame memory 122 (used in the same manner as theframe memory 103) that moves for the motion vector 105 as a predictedimage 106. The quantized orthogonal transformation coefficients 121 aredecoded by the inverse quantization unit 115 and the inverse adaptivetransformation unit 116 and added to the predicted image 106 to make thefinal decoded image 117. The transformation base operation unit 118calculates and outputs the same transformation bases 119 as the imageencoding apparatus. The inverse adaptive transformation unit 116, usingthe transpose, inversely converts the transformation coefficients intothe signal in the image space. The decoded image 117 is output to thedisplay device at a predetermined timing to reproduce the image.

The second embodiment of the present invention will be described below.

The image encoding apparatus according to this embodiment is constructedas showed in FIG. 11, for example, and the image decoding apparatusaccording to this embodiment is constructed as showed in FIG. 12, forexample. This embodiment reduces the redundancy remaining in the timedirections by the motion compensation inter-frame prediction, andmodifies the DCT bases so that the base corresponding to the principalcomponent can capture the waveform pattern of the predicted image of amacro block obtained by the motion compensation prediction to compressthe information by encoding using the modified bases. Several sets oftransformation bases that fit local characteristics of correspondingimage are prepared, and a set of transformation bases that fits thepattern of the predicted image is selected. Since the same set oftransformation bases is provided to both the image encoding apparatusand the image decoding apparatus, it is not necessary to transmitinformation other than ID information indicating the switching of bases.The image decoding apparatus is required only to selects a set of basesbased on the ID information and does not need to calculate bases.

The image encoding apparatus showed in FIG. 11 employs the sameprocedure of the motion compensation inter-frame prediction as describedin connection with the conventional example. The procedure isillustrated in FIG. 3, and block matching processing to detect themotion vector is illustrated in FIG. 4. The procedure (FIGS. 6A and 6B)to quantize the coefficients obtained by the orthogonal transformationand to encode the coefficients into entropy codes are identical to theconventional example. The operation of the image encoding apparatus andthe image decoding apparatus is described by reference to thesedrawings.

In FIG. 11, an input image signal 201 is a signal of a framed image in atime series of framed images (the framed image to be encoded correspondsto the current frame of FIG. 3). The current frame is encoded in thefollowing procedure by a macro block. The current macro block istransferred to the motion detection unit 202 for detecting the motionvector 205. The motion compensation unit 207 retrieves a predicted image206 of each macro block by looking up partially decoded images 217 inthe frame memory 203 using the motion vector 205.

The predicted remainder signal 208 is obtained as the difference betweenthe current macro block and the predicted image 206 and converted intothe orthogonal transformation coefficient data 210 by the adaptivetransformation unit 209. The transformation bases 219 used by theadaptive transformation unit 209 is selected by the transformation baseoperation unit 218 depending on the used pattern of the predicted image206. The selected transformation bases 219 are transferred to theadaptive transformation unit 209 and used for the orthogonaltransformation. The ID information 250 of a transformation base 219 foreach orthogonal transformation processing is multiplexed on thecompressed stream 214 and transferred to the image decoding apparatus.The operation of the transformation base operation unit 218 will bedescribed later.

The orthogonal transformation coefficient data 210 obtained by theadaptive transformation unit 209 are quantized by the quantization unit211, encoded into run-length codes by the variable length encoding unit213, and multiplexed on the compressed stream 214 for transmission. Amotion vector 205 of each macro block is multiplexed on the compressedstream 214 and transmitted. The quantized coefficients 212 are partiallydecoded by the inverse quantization unit 215 and the inverse adaptivetransformation unit 216. The result is added to the predicted image 206to make the same decoded image 217 as the image decoding apparatus. Thedecoded image 217 is stored in the frame memory 203 as a partiallydecoded image to be used for the prediction of the next frame.

The transformation base operation unit 218 performs the followingoperation.

The transformation base operation unit 218 divides the input predictedimage 206 into regions (N×N pixel blocks, where N=4, 8, and so forth) towhich the orthogonal transformation is applied, obtain thetransformation bases 219 for each region, and output the transformationbases 219 to the adaptive transformation unit 109. As showed in FIG. 10,average brightness distributions x_(H) and x_(V) in the horizontal andvertical directions are obtained for each region of the predicted image206 to which the orthogonal transformation is applied. Waveform patternsthat reflect the principal components of each region in the horizontaland vertical directions are obtained (see FIG. 10). “K” kinds ofnormalized orthogonal bases A_(i) (i=0, 1, . . . , K-1), the principalaxis of which reflects the pattern of typical average brightnessdistribution vector x_(H) and x_(V) are prepared for the transform baseoperation unit 218, and one of the bases A_(i) corresponding to x_(H)and x_(V) is selected. Examples of the bases (N=4) prepared as A_(i) areshowed in FIGS. 13B through 19B.

In addition, inverse transformation matrices corresponding to thetranspose of the transformation bases used by the inverse adaptivetransformation unit 216 of the image decoding apparatus to be describedlater are showed in FIGS. 13B through 19B as well as the normalizedorthogonal bases (forward transformation matrix).

In the case of basic DCT base showed in FIGS. 13B and 13C, the principalaxis base is a direct current component. Using the forwardtransformation matrix that becomes this DCT base and the inversetransformation matrix, the DCT transformation and its inversetransformation represented by the following formulae showed in FIG. 13Aare performed. $\begin{matrix}{{{{Forward}\quad{transformation}\quad{F(u)}} = {\sqrt{\frac{2}{N}}{C(u)}{\sum\limits_{x = 0}^{N - 1}{{f(x)}\cos\quad\frac{{\pi\left( {{2\quad x} + 1} \right)}u}{2\quad N}}}}}{{Inverse}\quad{transformation}\quad{f(x)}\sqrt{\frac{2}{N}}{C(u)}{F(u)}\cos\frac{{\pi\left( {{2\quad x} + 1} \right)}u}{2\quad N}}\begin{matrix}{\quad{{C(u)} = {\frac{1}{\sqrt{2}}\quad\left( {u = 0} \right)}}} \\{1\quad\left( {u \neq 0} \right)}\end{matrix}} & \left( {{Formula}\quad 1} \right)\end{matrix}$In the case of the above DCT base, the brightness of patterns showed inFIGS. 14A and 14B and patterns showed in FIGS. 15A and 15B changessmoothly. Patterns showed in FIGS. 16A and 16B and patterns showed inFIGS. 17A and 17B have ups and downs of pixel values shaped likemountains and valleys in an N×N pixel block. Further, in the case ofpatterns showed in FIGS. 18A and 18B and patterns showed in FIGS. 19Aand 19B, the principal axis reflects a pattern having a steep edge. As anorm useful in selecting bases, the transformation base operation unit218 selects the transformation base A_(i) that maximizes the innerproduct between the average brightness distribution vectors x_(H), x_(V)and the principal axis base vector, for example.

In the case where there is similarity in pattern between the predictedimage and the image to be encoded (the predicted remainder signal), theconcentration ratio of the orthogonal transformation coefficients of theimage to be encoded is increased by selectively using a base in whichthe power concentration ratio of the pattern of the predicted image ishigh by the above procedure. Further, since the frequency in which IDinformation 250 is selected is biased due to the characteristics of animage, it is possible to reduce the bit amount of the ID information tobe transmitted by assigning an appropriate Huffman code by the variablelength encoding unit 213 and using entropy encoding such as arithmeticcode.

The image decoding apparatus showed in FIG. 12 operates as follows.

In this image decoding apparatus, a variable length decoding unit 220detects a sync word indicating the top of each frame in the receivedcompressed stream 214 and restores the transformation base IDinformation 250, the motion vector 205, the quantized orthogonaltransformation coefficient 221 for each orthogonal transformation andfor each macro block. The motion vector 205 is transferred to the motioncompensation unit 207. The motion compensation unit 207, in the samemanner as it operates in the image encoding apparatus, obtains a partialimage stored in the frame memory 222 (used in the same manner as theframe memory 203) that moves for the motion vector 205 as a predictedimage 206. The quantized orthogonal transformation coefficients 221 aredecoded by the inverse quantization unit 215 and the inverse adaptivetransformation unit 216 and added to the predicted image 206 to make thefinal decoded image 217.

In the transformation base storage unit 251, there are the same base setA_(i) (see FIGS. 13 through 19) as the image encoding apparatus. Atransformation base 219 is selected based on transformation base IDinformation 250, and the selected transformation base 219 is transferredto the inverse adaptive transformation unit 216. The inverse adaptivetransformation unit 216 inversely transforms the transformationcoefficient into a signal in an image space using the transpose of theselected transform base A_(i). The decoded image 217 is output to thedisplay device at a predetermined timing.

As a variation of the above second embodiment, it is possible totransmit only flag information indication which base to be used, thatis, whether to use the DCT base being the reference transformation base,or which one of the transformation bases A_(i) to be used, instead ofthe ID information 250, as is, identifying which transformation baseA_(i) (i=0, 1, . . . , K-1). In this case, the image encoding apparatusis constructed as showed in FIG. 20, for example, and the image decodingapparatus is constructed as showed in FIG. 21, for example.

The base operation unit 218A of the image encoding apparatus showed inFIG. 20 is provided with “K” kinds of normalized orthogonal base A_(i)(i=0, 1, . . . , K-1) that reflects typical image patterns as showed inFIGS. 13 through 19. This base operation unit 218A divides the inputpredicted image 206 into regions (N×N pixel blocks, where N=4, 8, and soforth) to which the orthogonal transformation is applied, and selectsthe most appropriate adaptive transformation base from thetransformation base A_(i) for each region to which the orthogonaltransformation is applied. As a method to select base A_(i), it ispossible, for example, to obtain the average brightness distributionvectors x_(H) and x_(V) in the horizontal directions and the verticaldirections, respectively, of each region of the predicted image to whichthe orthogonal transformation is applied and to select A_(i) thatmaximizes the inner product between the average brightness distributionvectors and the principal axis base vector.

The DCT base or the above adaptive transformation base adaptivelyobtained for the predicted image 206, whichever gives a higherefficiency of encoding, is selected and output as the transformationbase 219. When comparing the efficiency of encoding, one can selectwhichever minimizes a rate distortion cost defined as a linear sum ofencoding distortion and the amount of codes, for example. Thetransformation base operation unit 218A transmits flag information 250Aindicating which the DCT base or the base determined by thetransformation base operation unit 218A to the image decoding apparatusby multiplexing the flag information 250A to the compressed stream.

The transformation base 219 thus obtained is transmitted to the adaptivetransformation unit 209A and used for transformation.

In the image decoding apparatus showed in FIG. 21, the above flaginformation 250A is extracted from the compressed stream and input tothe transformation base operation unit 218B. If the transformation baseoperation unit 218B determines that a base other than DCT is used, thetransformation base operation unit 218B identifies a transformation baseA_(i) using the completely same standard as used by the image encodingapparatus and outputs it as the transformation base 219. If thetransformation base operation unit 218B determines that the DCT base isused, the transformation base operation unit 218B outputs the DCT baseas the transformation base 219.

The image decoding apparatus can use the completely same predicted image206 as used by the image encoding apparatus. It is possible to use thesame standard of judgment as described in connection with the aboveimage encoding apparatus. That is, one can obtain the average brightnessdistribution vectors x_(H) and x_(V) in the horizontal and verticaldirections of each region of the predicted image 206 to which theorthogonal transformation is applied and select a transformation baseA_(i) that maximizes the inner product between the average brightnessdistribution vectors and the principal axis base vector. Thetransformation base 219 thus obtained is used by the inverse adaptivetransformation unit 216 to reproduce the signal in the image space byinversely transforming the transformation coefficient.

Because an image signal is generally not steady, the more various thebase set A_(i) is, the higher the efficiency of the adaptive orthogonaltransformation becomes. According to the image encoding apparatus andthe image decoding apparatus described above, even if a great variety oftransformation bases A_(i) that fit image patterns, it is not necessaryto increase additional information to identify one of the transformationbases A_(i), which results in efficient encoding.

Further, as another variation of the second embodiment, it is possibleto have the receiving side automatically determine which the DCT basebeing the reference of transformation bases or the transformation baseA_(i) based on the activity (for example, the divergence of brightnessand the absolute difference between the maximum and minimum ofbrightness) of a region of the reference image instead of transmittingthe flag information indicating which the DCT base or the transformationbase A_(i). If the activity is high, the transformation base IDinformation is transmitted by the encoding side, and if the activity islow, the receiving side uses the DCT base instead of transmitting thetransformation base ID information. If the activity of the referenceimage region is higher than a predetermined value, the receiving sidedetermines that a transformation bases is designated, and decodes thereceived transformation base ID information.

The third embodiment of the present invention will be described below.

The image encoding apparatus according to this embodiment is constructedas showed in FIG. 22, for example, and the image decoding apparatusaccording to this embodiment is constructed as showed in FIG. 23, forexample. In this embodiment, the image encoding apparatus transmits thetransformation base obtained thereby as encoded data to the imagedecoding apparatus so that the image decoding apparatus does not need tocalculate the base.

In the image encoding apparatus showed in FIG. 22, an input image signal301 is a signal of frame image in a time series of frame images (Theframe image to be encoded corresponds to the current frame of FIG. 3).The current frame is encoded by a macro block in the followingprocedure. The current macro block is transferred to the motiondetection unit 302, and the motion detection unit 302 detects the motionvector 305. The motion compensation unit 307 retrieves a predicted image306 of each macro block by reference to the partially decoded image 317stored in the frame memory 303 using the motion vector 305.

The predicted remainder signal 308 is obtained as the difference betweenthe current macro block and the predicted image 306, and transformedinto the orthogonal transformation coefficient data 310 by the adaptivetransformation unit 309. The transformation base 319 that the adaptivetransformation unit 309 uses is generated by the transformation baseoperation unit 318 depending on the pattern of the used predicted image306. Since the same transformation base is used at the decoding side,each transformation base vector of the transformation base 319 isencoded and multiplexed on the compressed stream 314. The transformationbase 319 is also transferred to the adaptive transformation unit 309 andused for the orthogonal transformation. The operation of thetransformation base operation unit 318 is exactly identical to that ofthe transformation base operation unit 118 according to the firstembodiment described above.

The orthogonal transformation coefficient data 310 obtained by theadaptive transformation unit 309 are quantized by the quantization unit311, scanned and encoded into run-length codes by the variable lengthencoding unit 313, and multiplexed on the compressed stream 314 fortransmission. The motion vector 305 is multiplexed on the compressedstream 314 by a macro block and transmitted. The quantized coefficient312 is partially decoded by the inverse quantization unit 315 and theinverse adaptive transformation unit 316. The result is added to thepredicted image 306 to generate the same decoded image 317 as the imagedecoding apparatus. Because the decoded image 317 is used for theprediction of the next frame, the decoded image 317 is stored in theframe memory 303 as a partially decoded image.

In the image decoding apparatus showed in FIG. 23, in response toreception of the compressed stream 314, the variable length decodingunit 320 detects a sync word indicating the top of each frame andreproduces the transformation base 319 used for each orthogonaltransformation, the motion vector 305, the quantized orthogonaltransformation coefficient 321. The motion vector 305 is transferred tothe motion compensation unit 307. The motion compensation unit 307, inthe same manner in which the motion compensation unit of the imageencoding apparatus operates, retrieves a portion of image that has movedfor the motion vector 305 as the predicted image 306 from the framememory 322 (used in the same manner in which the frame memory 303 isused). The quantized orthogonal transformation coefficient 321 isdecoded through the inverse quantization unit 315 and the inverseadaptive transformation unit 316, and becomes the final decoded image317 by being added to the predicted image 306. The inverse adaptivetransformation unit 316 inversely transforms the transformationcoefficient using the transpose of the transformation base 319 toreproduce a signal in an image space. The decoded image 317 is output tothe display device at a predetermined timing to reproduce the image.

Additionally, the fourth embodiment of the present invention will bedescribed below.

The image encoding apparatus according to this embodiment is constructedas showed in FIG. 24, for example, and the image decoding apparatusaccording to this embodiment is constructed as showed in FIG. 25, forexample.

This embodiment encodes an image using a base set A_(i) (i=0, 1, . . . ,K-1) by adaptively selecting a transformation base in the same manner inwhich the second embodiment described above operates, and additionallyupdates the transformation base A_(i) dynamically. Accordingly, when theimage encoding apparatus encounters an image pattern with which theimage encoding apparatus cannot fully comply using the fixedtransformation set, the image encoding apparatus can further improve theefficiency of encoding.

In the image encoding apparatus showed in FIG. 24, the input imagesignal 401 represents a signal of each frame image in a time series offrame images (the frame image to be encoded corresponds to the currentframe of FIG. 3). The current frame is encoded by a macro block in thefollowing procedure. The current macro block is transferred to themotion detection unit 402, and the motion detection unit 402 detects themotion vector 405. The motion compensation unit 407 retrieves thepredicted image 406 of each macro block from the frame memory 403 byreference to partially decoded image 417 using the motion vector 405.

The predicted remainder signal 408 is obtained as the difference betweenthe current macro block and the predicted image 406, and converted intothe orthogonal transformation coefficient data 410 by the adaptivetransformation unit 409. The transformation base 419 that is used by theadaptive transformation unit 409 is adaptively selected by thetransformation base operation unit 418 depending on the pattern of thepredicted image 406. The selected transformation base 419 is transferredto the adaptive transformation unit 409 and used for the orthogonaltransformation. In addition, the ID information 450 of thetransformation base 419 by an orthogonal transformation operation ismultiplexed on the compressed stream 414 and transmitted to the imagedecoding apparatus.

Further, when the transformation base operation unit 418 generatesanother transformation base that is no included in the base set A_(i) atthe point of time, the transformation base is multiplexed on thecompressed stream 414 through the variable length encoding unit 413 andtransmitted with the ID information 450. In this case, the IDinformation 450 that is transmitted means the ID information of a basethat is replaced by the transformation base that is transmitted at thesame time. The operation of the transformation base operation unit 418will be described later.

The orthogonal transformation coefficient data 410 obtained by theadaptive transformation unit 409 is quantized by the quantization unit411, scanned and encoded into run-length codes by the variable lengthencoding unit 413, and multiplexed on the compressed stream fortransmission. The motion vector 405 is also multiplexed on thecompressed stream 414 by a macro block and transmitted. The quantizedcoefficient 412 is partially decoded by the inverse quantization unit415 and the inverse adaptive transformation unit 416. The same decodedimage 417 as that of the image decoding apparatus is generated by addingthe partially decoded image and the predicted image 406. Since thedecoded image 417 is used for the prediction of the next frame, thedecoded image 417 is stored in the frame memory 403 as the partiallydecoded image.

The transformation base operation unit 418 operates as follows.

The transformation base operation unit 418 divides the input predictedimage 406 into regions (N×N pixel blocks, where N=4, 8, and so forth) towhich the orthogonal transformation is applied, obtains a transformationbase 419 by a region, and outputs the obtained transformation base tothe adaptive transformation unit 409. The transformation base operationunit 418 first obtains average brightness distribution x_(H) and x_(V)in the horizontal and vertical directions for each region of thepredicted image 406 to which the orthogonal transformation is applied. Awaveform pattern that reflects the principal components of thehorizontal and vertical directions of each region is obtained (see FIG.10). “K” kinds of normalized orthogonal bases A_(i) (i=0, 1, . . . ,K-1) of which principal axis reflects the pattern of the typical averagebrightness distribution vectors x_(H) and x_(V) are prepared in thetransformation base operation unit 418, and one of the bases A_(i)corresponding to x_(H) and x_(V) is selected. An example of base A_(i)(N=4) is showed in FIGS. 13 through 19. Since each example is describedin detail in connection with the second embodiment, their description isomitted here.

As described in connection with the first embodiment, the transformationbase operation unit 418 calculates a base (named A′) using x_(H) andx_(V). The transformation base operation unit 418 selects a base thatmaximizes the inner product between the average brightness distributionvector x_(H), x_(V) and the principal base vector from A_(i) (i=0, 1, .. . , K-1) and A′. When A′ is selected, a base of which inner product(similarity information) is the smallest is replaced with A′. In thecase where A′ is selected, the amount of transmitted codes increasessince the base A′ needs to be transmitted. In consideration of theincrease of the amount of transmitted codes, it is possible to selectthe best one of the bases A_(i) and compare it with A′ by encoding theimage using both bases to select one that shows better balance of rateand distortion. It is also possible to give the inner product(similarity information) an offset so that one of bases A_(i) is likelyto be selected.

In the case where there is similarity in pattern between the predictedimage and the image to be encoded (predicted remainder signal), it ispossible to improve the concentration ratio of the orthogonaltransformation coefficient of the image to be encoded by selectivelyusing a base that increases the power concentration ratio of the patternof the predicted image. Since the frequency at which each item of the IDinformation 450 becomes biased depending on the characteristics of theimage, it is possible to reduce the bit amount of ID information that istransmitted by appropriately assigning Huffman code or using entropycodes such as an arithmetic code. Additionally, since the base that isreplaced by A′ is uniquely determined based on the ID information 450,it is possible to reduce the amount of base information to betransmitted by transmitting only a base vector of A′ different from thatof the base to be replaced with A′.

The image decoding apparatus showed in FIG. 25 operates as follows.

In this image decoding apparatus, in response to reception of thecompressed stream 414, the variable length decoding unit 420 detects async word indicating the top of each frame and then, decodes thetransformation base ID information 450 used for each orthogonaltransformation, the transformation base 419 required in the case where abase is replaced, the motion vector 405, and the quantized orthogonaltransformation coefficient 421. The motion vector 405 is transferred tothe motion compensation unit 407, and the motion compensation unit 407retrieves a portion of image that is moved for the motion vector 405from the frame memory 422 (used in the same manner in which the framememory 403 is used) as the predicted image 406. The quantized orthogonaltransformation coefficient 421 is decoded by the inverse quantizationunit 415 and the inverse adaptive transformation unit 416, and then,added to the predicted image 406 to make the final decoded image 417.

A base 419 corresponding to the transformation base ID information 450is selected from the same base set A_(i) stored in the transformationbase storage unit 418, and transferred to the inverse adaptivetransformation unit 416. In the case where the transformation base 419has been transmitted as encoding data, however, a base in the base setsA_(i) corresponding to the ID information 450 is replaced with thetransformation base 419, and the transformation base 419 is transferred,as is, to the inverse adaptive transformation unit 416. The inverseadaptive transformation unit 416 inversely transforms the transformationcoefficient using the transpose of the selected base and reproduces asignal in the image space. The decoded image 417 is output to thedisplay device at a predetermined timing to reproduce the image.

The fifth embodiment of the present invention will be described below.

The image encoding apparatus according to this embodiment is constructedas showed in FIG. 26, for example, and the image decoding apparatusaccording to this embodiment is constructed as showed in FIG. 27, forexample. In this embodiment, an optimum transformation base for thepredicted image is obtained using a high correlation between thepredicted image obtained by the motion compensation inter-frameprediction and the image to be encoded (the current macro block), andthe optimum transformation base is directly applied to the image to beencoded. That is, an intra-frame signal, instead of the predictedremainder signal, is directly encoded by the orthogonal transformation.Accordingly, since the transformation coefficients of the current macroblock signal are concentrated near the principal axis, even theintra-frame signal can encode the image to be encoded with a highefficiency. Additionally, the predicted image is common to both theimage encoding apparatus and the image decoding apparatus, and bothapparatuses can generate the orthogonal transformation base followingthe same procedure. It is not necessary to transmit the base data.

In the image encoding apparatus showed in FIG. 26, the input imagesignal 501 indicates the signal of a frame image in a time series offrame images (the frame image that is an object of encoding correspondsto the current frame of FIG. 3). The current frame is encoded by a macroblock by the following procedure. The current macro block is transferredto the motion detection unit 502 that detects the motion vector 505. Themotion compensation unit 507 retrieves the predicted image 506 of eachmacro block from the partially decoded image 517 stored in the framememory 503.

What is different from the other embodiments is that, in thisembodiment, the predicted image 506 is transmitted, without beingsubtracted from the current macro block, to the transformation baseoperation unit 518 and used for the generation of the transformationbase 519 that is used to encode the current macro block.

The transformation base operation unit 518 generates a KL(Karhunen-Loeve) transformation base using the predicted image 506 as asource. KL transformation generates the optimum, from the standpoint ofpower concentration, normalized orthogonal transformation from a signalthat complies with stationary probability process. Accordingly, in thecase of an image signal that is not stationary, it is necessary toobtain the KL transformation base for each transformation and transmitthe KL transformation base to the image decoding apparatus so that theimage decoding apparatus can use the same base. Since the KLtransformation is obtained based on the predicted image, it is notnecessary to transmit the KL transformation base to the image decodingapparatus. In general, the predicted image 506 is extracted as a patternthat is similar to the current macro block based on the motioncompensation inter-frame prediction algorithm. That is, it is probablethat the signal distribution of the predicted image 506 is substantiallysimilar to that of the current macro block. From this standpoint, if theKL transformation based on the predicted image is used instead of DCT,the power concentration of the transformation coefficient of the currentmacro block can be increased. In other words, the signal can berepresented by fewer transformation coefficients.

The current macro block is converted into orthogonal transformationcoefficient data 510 by the adaptive transformation unit 509 using theKL transformation base of the predicted image 506. The orthogonaltransformation coefficient data 510 is quantized by the quantizationunit 511, scanned and encoded into a run-length code by the variablelength encoding unit 513, and multiplexed on the compressed stream fortransmission. The motion vector 505 is also multiplexed on thecompressed stream 514 by a macro block and transmitted. The quantizedcoefficient 512 is partially decoded through the inverse quantizationunit 515 and the inverse adaptive transformation unit 516, and thereproduced image 517 that is identical to the reproduced image of theimage decoding apparatus is generated. The reproduced image 517 isstored in the frame memory 503 and used for the prediction of the nextframe.

The image decoding apparatus showed in FIG. 27 operates in the followingmanner.

In the case of this image decoding apparatus, in response to receptionof the compressed stream 514, the variable length decoding unit 520detects a sync word indicating the top of each frame, and then,reproduces the motion vector 505 and the quantized orthogonaltransformation coefficient 521 of each macro block. The motion vector505 is transferred to the motion compensation unit 507. The motioncompensation unit 507, in the same manner in which that of the imageencoding apparatus, retrieves a portion of image that has moved for themotion vector 505 from the frame memory 522 (used in the same manner asthe frame memory 503 is used) as the predicted image 506.

The quantized orthogonal transformation coefficient 521 is decodedthrough the inverse quantization unit 515 and the inverse adaptivetransformation unit 516, and makes the decoded image 517. Thetransformation base operation unit 518, in the same manner of the imageencoding apparatus, obtains the KL transformation base using thepredicted image 506 as a source and outputs it as the transformationbase 519. The inverse adaptive transformation unit 516 performs aninverse transformation on the transformation coefficient based on thetransformation base 519 and reproduces a signal in the image space. Thedecoded image 517 is output to the display device at a predeterminedtiming and displayed on it.

Further the six embodiment of the present invention will be describedbelow.

The image encoding apparatus according to this embodiment is constructedas showed in FIG. 28, for example, and the image decoding apparatusaccording to this embodiment is constructed as showed in FIG. 29, forexample. This embodiment relates to an apparatus that encodes anddecodes an image by encoding method to which a technique called“Matching Pursuits” is applied and introduces an adaptive basereflecting the signal pattern of the predicted image as described inconnection with the previous embodiments. According to “MatchingPursuits”, an image signal f that is the object of encoding can berepresented as the following formula 2 using an over-complete base setg_(k) provided in advance. $\begin{matrix}{f = {{\sum\limits_{n = 0}^{m - 1}{\left\langle {{R_{n}f},g_{k_{n}}} \right\rangle g_{k_{n}}}} + {R_{m}f}}} & \left( {{Formula}\quad 2} \right)\end{matrix}$where “n” is the number of base search steps; “R_(n)f” is a signal forwhich a base is searched in the nth search step (hereinafter, called thenth partial signal waveform); and “g_(kn)” is a base that maximizes aninner product with R_(n)f. “R_(m)f” is a remaining component for which abase is searched in the m^(th) search step. That is, the more the numberof steps is, the more accurate the signal f is represented. The signalthat is the object of the n+1^(th) search step is as follows.R _(n) f−<R _(n) f,g _(k) _(n) >g _(k) _(n)   (Formula 3)This means that the more the number of bases is, the more accurately thesignal f can be represented. “R_(n)F” is a signal waveform defined in apredetermined window range centered at an arbitral position in theimage. The following information is encoded for each search step: anindex indicating “g” (since “g_(k)” is commonly used by the encodingside and the decoding side, only an index needs to be exchanged toidentify a base), an inner product (similarity information)<R_(n)f,g_(k) _(n) >  (Formula 4)(corresponding to a base coefficient), position information p=(x_(k),y_(k)) in a screen of the partial signal waveform R_(n)f.

According to this representation of the image signal and the encodingmethod, the more the number of bases is, that is, the more the number ofsearch steps is, the more the amount of code becomes, and the less thedistortion becomes.

In the image encoding apparatus showed in FIG. 28, an input image signalis a signal of a frame image in a time series of image frames (the frameimage that is the object of encoding corresponds to the current frame ofFIG. 3). The current frame is encoded in the following procedure. Thecurrent frame is transferred to the motion detection unit 602, and themotion vector 605 is detected by a macro block. The motion compensationunit 607 retrieves the predicted image 606 of the current frame from thepartially decoded image 604 stored in the frame memory 603 using themotion vector 605. The predicted remainder signal 608 is obtained as thedifference between the predicted image 606 and the current frame (theinput image signal 601).

Subsequently, the base search unit 609 generates a base parameter(hereinafter referred to as “atom”) 610 for the predicted remaindersignal 608 based on the above “Matching Pursuits” algorithm. The baseset g_(k) is stored in a base codebook 619. If one can find a base thatcan accurately represent the partial signal waveform in the initialsearch step, the one can represent the partial signal waveform with afew atoms, that is, a small amount of codes, to such an extent that thebase accurately represent the partial signal waveform. In addition,because the over-complete base g_(k) is used from the initial stage, anyvector of which waveform pattern satisfies linearly independent from thevectors included in g_(k) and of which a norm is one (1) can be used asa new base.

Accordingly, this embodiment is constructed so that a waveform patternof image signals included in the predicted image 606 can be used as anew base. As described above, the pattern of the predicted image signalsometimes substantially correlates to that of the predicted remaindersignal; in the case where the motion compensation prediction fails inthe outline region of an object, an edge pattern similar to thepredicted image appears in the predicted remainder signal, for example.Thus, if a base is generated from the predicted image, the number ofpotentially usable bases increases, and the predicted remainder signalcan be efficiently represented.

Specifically, the base operation unit 618 generates a candidate of a newbase h_(j) 652.h_(j)=P_(j)l|P_(j)|  (Formula 5)where “P_(j)” is a waveform vector generated from the partiallypredicted image, and “|P_(j)|” is a norm of P_(j). The partiallypredicted image means a partial signal waveform that is located at thesame position as the partial signal waveform of the object of basesearch. Since the partial predicted image is located at the sameposition in the screen as the position information of an atom that is tobe encoded, the image decoding apparatus does not need additionalinformation to identify the position of the partial predicted image. Thefollowing may be used as P_(j):

-   1) a waveform pattern obtained by subtracting DC component from the    partial predicted image;-   2) a waveform pattern obtained by extracting edge component from the    partial predicted image (extracted by effecting a Sobel operator,    for example, in the horizontal or vertical directions);-   3) a difference waveform pattern between the partially predicted    image and a pattern to be obtained by horizontally shifting the    partially predicted image by ¼ pixel;-   4) a difference waveform pattern between the partially predicted    image and a pattern to be obtained by vertically shifting the    partial predicted image by ¼ pixel;-   5) a difference waveform pattern between the partial predicted image    and a pattern to be obtained by horizontally shifting the partial    predicted image by ½ pixel;-   6) a difference waveform pattern between the partial predicted image    and a pattern to be obtained by vertically shifting the partial    predicted image by ½ pixel; and-   7) a waveform pattern obtained by smoothing the partial predicted    image.

A new base set h_(j) is generated by the formula 5 using P_(j) based onthe partial predicted image. Since h_(j) is generated using only signalsincluded in the predicted image 606, it is not necessary to transmitbase vectors. One needs to transmit only the index of h_(j) instead ofg_(k). It is possible to increase the candidates for a new base withouttransmitting additional amount of codes.

Flag information 650 for identifying whether to use g_(k) or h_(j) maybe encoded.

Though it is not showed in the drawings, if one desires to use an h_(j)that fits a partial signal waveform with other arbitral partial signalwaveforms, the one can replace a base included in g_(k) that is not usedfrequently with the h_(j). According to the above procedure, the basesearch unit 609 outputs an atom parameter 610 including an index ofg_(k) or h_(j), an inner product of the partial signal waveform and thebase, and position of the partial signal waveform, in addition to theflag information 650, to the base encoding unit 611. The base encodingunit 611 quantizes the atom parameter 610, transfers the encoded data tothe variable length encoding unit 613, and additionally, inputs theencoded data to the base decoding unit 616. The base decoding unit 616reproduces the image signal using a base pattern encoded from g_(k) orh_(j) that are switched by flag information 650 and the switch 651.Subsequently, the reproduced image signal is added to the predictedimage 606 to generate the partially decoded image 617, which is storedin the frame memory 603 and used for the compensation prediction of thenext frame.

The image decoding apparatus showed in FIG. 29 operates in the followingmanner.

In this image decoding apparatus, in response to reception of thecompressed stream 614, the variable length decoding unit 620 detects async word indicating the top of each frame, and reproduces the motionvector 605 and the atom parameter 621 by a macro block. The motionvector 605 is transferred to the motion compensation unit 607, and themotion compensation unit 607 retrieves an image portion that moved forthe motion vector 605 from the frame memory 622 (used as the framememory 603) as the predicted image 606.

The atom parameter 621 is decoded by the base decoding unit 616. A baseto be used for decoding is determined based on the flag information 650by switching the base codebook g_(k) 619 originally provided and thebase h_(j) generated from the predicted image 606. When h_(j) is used,the base operation unit 618 generates h_(j) from the predicted image 606complying with the same rule as the encoding side.

The output of the base decoding unit 616 is added to the predicted image606 to make the decoded image 617. The decoded image 617 is stored inthe frame memory 622 and used for the compensation of upcoming frames.The decoding image 617 is output to the display device at apredetermined timing to reproduce the image.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention claimed in theclaims 1-107, a signal that is an object to be encoded can betransformed and encoded using a transformation base matching to acharacteristic of the signal, and the encoded signal can be transformedusing the same transformation base after decoding the encoded signal. Asa result, the encoding and decoding of a signal becomes more efficient.

1. A method of encoding an object signal in compliance with atransformation rule, comprising: a first step of obtaining a referencesignal correlating to said object signal; a second step of deriving atransformation base that forms said transformation rule based on acharacteristic of the obtained reference signal; a third step ofencoding said object signal in compliance with the transformation ruleformed by the derived transformation base; and wherein the encodedobject signal is transmitted without the derived transformation base. 2.The method as claimed in claim 1, wherein said reference signal isidentical to a signal that is to be obtained when the object signalencoded by the method is decoded.
 3. The method as claimed in claim 1,wherein said object signal is an image signal indicating informationabout an image.
 4. The method as claimed in claim 3, wherein said imagesignal is a predicted remainder signal obtained by motion compensationprediction method from an input original image signal.
 5. The method asclaimed in claim 3, wherein said reference signal is a predicted imagesignal obtained by motion compensation prediction from said inputoriginal image signal.
 6. The method as claimed in claim 1, wherein saidtransformation base is generated based on a characteristic of saidreference signal.
 7. The method as claimed in claim 6, wherein, in saidsecond step, a new transformation base is generated by modifying atransformation base predetermined as a reference based on acharacteristic in signal value distribution of said reference signal. 8.The method as claimed in claim 3, wherein, in said second step, a newtransformation base is generated by modifying DCT transformation basebased on a characteristic in brightness distribution of a referencesignal correlated to said image signal.
 9. The method as claimed inclaim 8, wherein, in said second step, a relationship between saidbrightness distribution of reference signal and said DCT transformationbase is obtained; and said new transformation base is generated bymodifying said DCT transformation base based on said relationship. 10.The method as claimed in claim 9, wherein, in said second step, aplurality of waveform patterns that can appear as a characteristic inbrightness distribution of a reference signal are prepared; one of saidplurality of waveform patterns is selected in accordance with asimilarity with a characteristic in brightness distribution of saidreference signal; and a new transformation base is generated bymodifying DCT transformation based on a relationship between theselected waveform pattern and DCT transformation base.
 11. The method asclaimed in claim 1, wherein, in said second step, a transformation baseto be used in said third step is selected from a plurality ofpredetermined transformation bases based on a characteristic of saidreference signal.
 12. The method as claimed in claim 11, wherein, insaid second step, a relationship between each one of the plurality ofpredetermined transformation bases and a characteristic in signal valuedistribution of said reference signal is obtained; and a transformationbase of which a characteristic in signal value distribution satisfies apredetermined standard is selected.
 13. The method as claimed in claim3, wherein, in said second step, a similarity between a base vectorindicating a characteristic of each one of a plurality of predeterminedtransformation bases and a characteristic in brightness distribution ofa reference signal correlating to said image signal; and atransformation base having said base vector of which similarity withsaid characteristic in brightness distribution of said reference signalsatisfies a predetermined standard is selected from the plurality oftransformation bases.
 14. The method as claimed in claim 13, wherein theplurality of transformation bases includes DCT base; and in said secondstep, either a transformation base selected from the plurality oftransformation bases other than DCT base based on said similarity or DCTbase whichever exhibits better efficiency in encoding is selected. 15.The method as claimed in claim 1, wherein, in said second step, saidtransformation base is generated based on said reference signal; and atransformation base to be used in said third step is selected from aplurality of transformation bases and the generated transformation basebased on said characteristic of said reference signal.
 16. The method asclaimed in claim 15, wherein, in the case where the generatedtransformation base is selected as said transformation base to be usedin said third step, the generated transformation base is added to saidplurality of transformation bases.
 17. The method as claimed in claim16, wherein a transformation base determined based on saidcharacteristic of said reference signal and a mutual relationship withanother one of said plurality of transformation bases is deleted fromsaid plurality of transformation bases.
 18. The method as claimed inclaim 3, wherein, in said second step, a new transformation base isgenerated by modifying DCT transformation base based on a characteristicof brightness distribution of said reference signal correlated to saidimage signal; a similarity between a base vector and said characteristicof brightness distribution of said reference signal is obtained, saidbase vector indicating a feature of each one of the plurality ofpredetermined transformation bases and the generated transformationbase; and a transformation base having said base vector of whichsimilarity with said characteristic of brightness distribution of saidreference signal satisfies a first standard is selected from saidplurality of transformation bases and the generated transformation baseto use in said third step.
 19. The method as claimed in claim 18,wherein, in the case where the generated transformation base is selectedas a transformation base to be used in said third step, the generatedtransformation step is added to said plurality of transformation bases;and a transformation base having said base vector said similarity ofwhich with said characteristic of brightness distribution of saidreference signal satisfies a second standard is deleted from saidplurality of transformation bases.
 20. The method as claimed in claim15, wherein one of said plurality of transformation bases is selectedbased on said characteristic of said reference signal; and said objectsignal is encoded using the selected one of said plurality oftransformation bases and the generated transformation base, and eitherthe selected one of said plurality of transformation bases or thegenerated transformation base is selected based on results of encoding.21. The method as claimed in claim 15, wherein, when a transformationbase to be used in said third step is selected from said plurality oftransformation bases and the generated transformation base, one of saidplurality of transformation bases is prioritized.
 22. The method asclaimed in claim 15, wherein, in the case where the generatedtransformation base is selected, the generated transformation base isencoded with said object signal.
 23. The method as claimed in claim 16,wherein the generated transformation base and information to identifythe deleted one of said plurality of transformation bases are encodedwith said object signal.
 24. The method as claimed in claim 3, whereinsaid reference signal is a predicted image signal obtained from inputoriginal image signal by motion compensation prediction; and the inputoriginal image signal makes said object signal.
 25. The method asclaimed in claim 24, wherein, in said second step, a Karhunen-Loevetransformation base obtained from a predicted image being a referencesignal as a source is generated as a transformation base to be used insaid third step.
 26. The method as claimed in claim 1, wherein a partialsignal waveform of said object signal is identified, and said partialsignal waveform is converted into similarity information indicating asimilarity between said partial signal waveform and a waveform vectormaking a transformation vector; information to identify said waveformvector, said similarity information, and a position of said partialsignal waveform in said object signal are encoded; and in said secondstep, a waveform vector that makes a transformation base is generatedbased on a characteristic of a partial signal of said reference signalcorresponding to said partial signal waveform of said object signal. 27.The method as claimed in claim 26, wherein a waveform vector to be usedas a transform base is selected based on a similarity relationshipbetween each waveform vector included in a predetermined group of wavevectors and the generated waveform vector and said partial signalwaveform.
 28. The method as claimed in claim 26, wherein said similarityinformation is based on an inner product between said partial signalwaveform and a waveform vector.
 29. The method as claimed in claim 26,wherein information identifying said waveform vector to be encodedincludes flag information indicating which the waveform vector includedin the group of waveform vectors or the generated waveform vector isselected.
 30. An apparatus for encoding an object signal in compliancewith a transformation rule, comprising: a first unit that obtains areference signal correlating to said object signal; a second unit thatderives a transformation base that forms said transformation rule basedon a characteristic of the obtained reference signal; a third unit thatencodes said object signal in compliance with the transformation ruleformed by the derived transformation base; and wherein the object signalis transmitted without the derived transformation base.
 31. Theapparatus as claimed in claim 30, wherein said reference signal isidentical to a signal that is to be obtained when the object signalencoded by the apparatus is decoded.
 32. The apparatus as claimed inclaim 30, wherein said object signal is an image signal indicatinginformation about an image.
 33. The apparatus as claimed in claim 32,wherein said image signal is a predicted remainder signal obtained bymotion compensation prediction method from an input original imagesignal.
 34. The apparatus as claimed in claim 32, wherein said referencesignal is a predicted image signal obtained by motion compensationprediction from said input original image signal.
 35. The apparatus asclaimed in claim 30, wherein said second unit generates saidtransformation base based on a characteristic of said reference signal.36. The apparatus as claimed in claim 35, wherein said transformationbase generation unit generates a new transformation base by modifying atransformation base predetermined as a reference based on acharacteristic in signal value distribution of said reference signal.37. The apparatus as claimed in claim 32, wherein said second unitgenerates a new transformation base by modifying DCT transformation basebased on a characteristic in brightness distribution of a referencesignal correlated to said image signal.
 38. The apparatus as claimed inclaim 30, wherein, said second unit selects a transformation base to beused by said third unit from a plurality of predetermined transformationbases based on a characteristic of said reference signal.
 39. Theapparatus as claimed in claim 38, wherein said base selecting unitobtains a relationship between each one of the plurality ofpredetermined transformation bases and a characteristic in signal valuedistribution of said reference signal and selects a transformation baseof which a characteristic in signal value distribution satisfies apredetermined standard.
 40. The apparatus as claimed in claim 32,wherein said second unit obtains a similarity relationship between abase vector indicating a characteristic of each one of a plurality ofpredetermined transformation bases and a characteristic in brightnessdistribution of a reference signal correlating to said image signal; andsaid second unit selects a transformation base having said base vectorof which similarity with said characteristic in brightness distributionof said reference signal satisfies a predetermined standard from theplurality of transformation bases.
 41. The apparatus as claimed in claim40, wherein the plurality of transformation bases includes DCT base; andsaid base selecting unit selects either a transformation base selectedfrom the plurality of transformation bases other than DCT base based onsaid similarity or DCT base whichever exhibits better efficiency inencoding.
 42. The apparatus as claimed in claim 30, wherein said secondunit further comprises: base generating unit that generates saidtransformation base based on a characteristic of said reference signal;and base selecting unit that selects a transformation base to be used bysaid third unit from a plurality of transformation bases and thegenerated transformation base based on said characteristic of saidreference signal.
 43. The apparatus as claimed in claim 42, wherein, inthe case where the generated transformation base is selected as saidtransformation base to be used by said third unit, the generatedtransformation base is added to said plurality of transformation bases.44. The apparatus as claimed in claim 43, wherein a transformation basedetermined based on said characteristic of said reference signal and amutual relationship with another one of said plurality of transformationbases is deleted from said plurality of transformation bases.
 45. Theapparatus as claimed in claim 32, wherein said second unit furthercomprises: base generating unit that generates a new transformation baseby modifying DCT transformation base based on a characteristic ofbrightness distribution of said reference signal correlated to saidimage signal; base selecting unit that obtains a similarity relationshipbetween a base vector and said characteristic of brightness distributionof said reference signal, said base vector indicating a feature of eachone of the plurality of predetermined transformation bases and thegenerated transformation base, and selects a transformation base havingsaid base vector of which similarity relationship with saidcharacteristic of brightness distribution of said reference signalsatisfies a first standard from said plurality of transformation basesand the generated transformation base to be used by said third unit. 46.The apparatus as claimed in claim 42, wherein said base selecting unitfurther comprises: a first unit that selects one of said plurality oftransformation bases based on said characteristic of said referencesignal; and a second unit that encodes said object signal using theselected one of said plurality of transformation bases and the generatedtransformation base, and selects either the selected one of saidplurality of transformation bases or the generated transformation basebased on results of encoding.
 47. The apparatus as claimed in claim 42,wherein said base selecting unit, when selecting a transformation baseto be used by said third unit from said plurality of transformationbases and the generated transformation base, prioritizes one of saidplurality of transformation bases.
 48. The apparatus as claimed in claim42, wherein, in the case where the generated transformation base isselected, the generated transformation base is encoded with said objectsignal.
 49. The apparatus as claimed in claim 43, wherein the generatedtransformation base and information to identify the deleted one of saidplurality of transformation bases are encoded with said object signal.50. The apparatus as claimed in claim 32, wherein said reference signalmakes a predicted image signal obtained from input original image signalby motion compensation prediction; and the input original image signalmakes said object signal.
 51. The apparatus as claimed in claim 50,wherein said second unit generates a Karhunen-Loeve transformation baseobtained from a predicted image making a reference signal as a source asa transformation base to be used by said third unit.
 52. The apparatusas claimed in claim 30, wherein a partial signal waveform of said objectsignal is identified, and said partial signal waveform is converted intosimilarity information indicating a similarity between said partialsignal waveform and a waveform vector making a transformation vector;information to identify said waveform vector, said similarityinformation, and a position of said partial signal waveform in saidobject signal are encoded; and said second unit further comprises basegenerating unit that generates a waveform vector that makes atransformation base based on a characteristic of a partial signal ofsaid reference signal corresponding to said partial signal waveform ofsaid object signal.
 53. The apparatus as claimed in claim 52, wherein awaveform vector to be used as a transform base is selected based on asimilarity relationship between each waveform vector included in apredetermined group of wave vectors and the generated waveform vectorand said partial signal waveform.
 54. The apparatus as claimed in claim52, wherein said similarity information is based on an inner productbetween said partial signal waveform and a waveform vector.
 55. Theapparatus as claimed in claim 52, wherein information identifying saidwaveform vector to be encoded includes flag information indicating whichthe waveform vector included in the group of waveform vectors or thegenerated waveform vector is selected.
 56. A method of decoding anencoded signal and transforming the decoded signal in compliance with atransformation rule to reproduce an image signal, comprising: a firststep of deriving a transformation base that forms said transformationrule based on the decoded signal; a second step of transforming thedecoded signal in compliance with said transformation rule based on thederived transformation base to reproduce said image signal; and whereinthe encoded signal is transmitted without the transformation base uponwhich the encoded signal has been transformed.
 57. The method as claimedin claim 56, wherein, in said first step, a signal correlated to thedecoded signal is obtained as a reference signal; and saidtransformation base is generated based on a characteristic of theobtained reference signal.
 58. The method as claimed in claim 57,wherein said reference signal is identical to a signal correlating to anobject signal before encoding.
 59. The method as claimed in claim 56,wherein said encoded signal is an encoded image signal where informationabout an image is encoded.
 60. The method as claimed in claim 59,wherein said encoded image signal making said encoded signal is anencoded predicted remainder signal obtained by encoding a predictedremainder signal obtained from said image signal by motion compensationprediction method.
 61. The method as claimed in claim 59, wherein saidreference signal is a predicted image signal obtained from the decodedimage signal by the motion compensation prediction method.
 62. Themethod as claimed in claim 57, wherein, in said first step, a newtransformation base is generated by modifying a transformation basesatisfying a predetermined standard based on a characteristic of signalvalue distribution of said reference signal.
 63. The method as claimedin claim 62, wherein said encoded signal makes an encoded image signalin connection with an image; in the first step, a new transformationbase is generated by modifying DCT transformation base based on acharacteristic of brightness distribution making a characteristic ofsignal value distribution of said reference signal.
 64. The method asclaimed in claim 63, wherein, in said first step, a relationship betweena characteristic of brightness distribution of said reference signal andDCT transformation base is obtained; a new transformation base isgenerated by modifying DCT transformation base based on saidrelationship.
 65. The method as claimed in claim 64, wherein, in saidfirst step, a plurality of waveform patterns that can appear as acharacteristic in brightness distribution of said reference signal arepredetermined; one of said plurality of waveform patterns is selected inaccordance with a similarity relationship with a characteristic inbrightness distribution of said reference signal; and a newtransformation base is generated by modifying DCT transformation basbased on a relationship between the selected waveform pattern and DCTtransformation base.
 66. The method as claimed in claim 56, wherein, insaid first step, a transformation base to be used in said second step isselected from a plurality of predetermined transformation bases based ona characteristic of said reference signal.
 67. The method as claimed inclaim 66, wherein, in said first step, a relationship between each ofsaid plurality of transformation bases and the characteristic of signalvalue distribution of said reference signal is determined; and atransformation base the determined relationship of which with thecharacteristic of signal distribution of said reference signal isselected.
 68. The method as claimed in claim 67, wherein said encodedsignal is an encoded image signal in connection with an image; in saidfirst step, a similarity relationship between a base vector indicatingsaid characteristic and said characteristic of signal value distributionof said reference signal is obtained for each one of said plurality oftransformation base; and a transformation base having said base vectorthe similarity relationship of which with said characteristic ofbrightness distribution of said reference signal satisfies apredetermined standard is selected from said plurality of transformationbases.
 69. The method as claimed in claim 56, wherein said encodedsignal makes an encoded original, image signal that is obtained byencoding an original image; said reference signal makes a predictedimage signal that is obtained by motion compensation prediction methodfrom said encoded original image signal; and in said first step, aKarhunen-Loeve transformation base source of which is said predictedimage signal making said reference signal is generated as atransformation base to be used in said second step.
 70. The method asclaimed in claim 56, wherein, in the case where information indicatingthat a waveform vector that makes a transformation base generated basedon a characteristic of a partial signal of a predetermined referencesignal has been used when an object signal is encoded, similarityinformation indicating a similarity between said waveform vector andsaid partial signal waveform of said object signal, and a position ofsaid partial signal waveform in said object signal are included in asignal that is obtained by decoding said encoded signal, in said firststep, a waveform vector making a transformation base, which is availablefrom said encoded signal, is generated based on a characteristic of saidpartial signal of said reference signal corresponding to a predeterminedreference signal used when said signal is encoded; in said second step,said partial signal waveform at the position in said object signal isreproduced by transforming said similarity information in compliancewith said transformation rule based on the generated waveform vector.71. The method as claimed in claim 70, wherein a signal that is obtainedby decoding said encoded signal includes flag information indicatingwhich a waveform vector or a predetermined waveform vector group isused, said waveform vector making a transformation base being generated,when said object signal is encoded, based on a characteristic of saidpartial signal of a predetermined reference signal; in said first step,in the case where said flag information indicates that said waveformvector group is used, a waveform vector identified by informationidentifying the used waveform vector is selected from said plurality ofwaveform vector groups; in said second step, said similarity informationis transformed in compliance with said transformation rule based on theselected waveform vector, and a partial signal waveform a said positionin said object signal is reproduced.
 72. An apparatus for decoding anencoded signal and transforming the decoded signal in compliance with atransformation rule to reproduce an image signal, comprising: a firstunit that derives a transformation base that forms said transformationrule based on the decoded signal; a second unit that transforms thedecoded signal in compliance with said transformation rule based on thederived transformation base to reproduce said image signal; and whereinthe encoded object signal is transmitted without the derivedtransformation base.
 73. The apparatus as claimed in claim 72, whereinsaid first unit obtains a signal correlated to the decoded signal as areference signal and generates said transformation base based on acharacteristic of the obtained reference signal.
 74. The apparatus asclaimed in claim 73, wherein said reference signal is identical to asignal correlating to an object signal before encoding.
 75. Theapparatus as claimed in claim 72, wherein said encoded signal is anencoded image signal where information about an image is encoded. 76.The apparatus as claimed in claim 75, wherein said encoded image signalmaking said encoded signal is an encoded predicted remainder signalobtained by encoding a predicted remainder signal obtained from saidimage signal by motion compensation prediction method.
 77. The apparatusas claimed in claim 75, wherein said reference signal is a predictedimage signal obtained from the decoded image signal by the motioncompensation prediction method.
 78. The apparatus as claimed in claim72, wherein said base generating unit generates a new transformationbase by modifying a transformation base satisfying a predeterminedstandard based on a characteristic of signal value distribution of saidreference signal.
 79. The apparatus as claimed in claim 78, wherein saidencoded signal makes an encoded image signal in connection with animage; said base generating unit generates a new transformation base bymodifying DCT transformation base based on a characteristic ofbrightness distribution making a characteristic of signal valuedistribution of said reference signal.
 80. The apparatus as claimed inclaim 79, wherein said base generating unit obtains a relationshipbetween a characteristic of brightness distribution of said referencesignal and DCT transformation base, and generates a new transformationbase by modifying DCT transformation base based on said relationship.81. The apparatus as claimed in claim 80, wherein said base generatingunit predetermines a plurality of waveform patterns that can appear as acharacteristic in brightness distribution of said reference signal,selects one of said plurality of waveform patterns in accordance with asimilarity relationship with a characteristic in brightness distributionof said reference signal, and generates a new transformation base bymodifying DCT transformation bas based on a relationship between theselected waveform pattern and DCT transformation base.
 82. The apparatusas claimed in claim 72, wherein said first unit further comprises a baseselecting unit that selects a transformation base to be used by saidsecond unit from a plurality of predetermined transformation bases basedon a characteristic of said reference signal.
 83. The apparatus asclaimed in claim 82, wherein said base selecting unit obtains arelationship between each of said plurality of transformation bases andthe characteristic of signal value distribution of said referencesignal, and selects a transformation base the relationship of which withthe characteristic of signal distribution of said reference signal. 84.The apparatus as claimed in claim 83, wherein said encoded signal is anencoded image signal in connection with an image; said base selectingunit obtains a similarity relationship between a base vector indicatingsaid characteristic and said characteristic of signal value distributionof said reference signal for each one of said plurality oftransformation base, and selects a transformation base having said basevector the similarity relationship of which with said characteristic ofbrightness distribution of said reference signal satisfies apredetermined standard from said plurality of transformation bases. 85.The apparatus as claimed in claim 72, wherein said encoded signal makesan encoded original image signal that is obtained by encoding anoriginal image; said reference signal makes a predicted image signalthat is obtained by motion compensation prediction method from saidencoded original image signal; and said first unit generates aKarhunen-Loeve transformation base source of which is said predictedimage signal making said reference signal as a transformation base to beused in said second step.
 86. The apparatus as claimed in claim 72,wherein, in the case where information indicating that a waveform vectorthat makes a transformation base generated based on a characteristic ofa partial signal of a predetermined reference signal has been used whenan object signal is encoded, similarity information indicating asimilarity between said waveform vector and said partial signal waveformof said object signal, and a position of said partial signal waveform insaid object signal are included in a signal that is obtained by decodingsaid encoded signal, said first unit generates a waveform vector makinga transformation base, which is available from said encoded signal,based on a characteristic of said partial signal of said referencesignal corresponding to a predetermined reference signal used when saidsignal is encoded; and said second unit reproduces said partial signalwaveform at the position in said object signal by transforming saidsimilarity information in compliance with said transformation rule basedon the generated waveform vector.
 87. The apparatus as claimed in claim86, wherein a signal that is obtained by decoding said encoded signalincludes flag information indicating which a waveform vector or apredetermined waveform vector group is used, said waveform vector makinga transformation base being generated, when said object signal isencoded, based on a characteristic of said partial signal of apredetermined reference signal; said first unit selects, in the casewhere said flag information indicates that said waveform vector group isused, a waveform vector identified by information identifying the usedwaveform vector from said plurality of waveform vector groups; and saidsecond unit transforms said similarity information in compliance withsaid transformation rule based on the selected waveform vector, andreproduces a partial signal waveform at said position in said objectsignal.
 88. A method of decoding an encoded signal and transforming thedecoded signal in compliance with a transformation rule to reproduce animage signal, comprising: a first step of deriving a transformation basethat forms said transformation rule based on the decoded signal; and asecond step of transforming the decoded signal in compliance with saidtransformation rule based on the derived transformation base toreproduce said image signal, wherein, in said first step, atransformation base to be used in said second step is selected from aplurality of predetermined transformation bases based on acharacteristic of said reference signal, in said first step, arelationship between each of said plurality of transformation bases andthe characteristic of signal value distribution of said reference signalis determined; and; a transformation base, the determined relationshipof which with the characteristic of signal distribution of saidreference signal is selected, said encoded signal is an encoded imagesignal in connection with an image; in said first step, a similarityrelationship between a base vector indicating said characteristic andsaid characteristic of signal value distribution of said referencesignal is obtained for each one of said plurality of transformationbase; a transformation base having said base vector the similarityrelationship of which with said characteristic of brightnessdistribution of said reference signal satisfies a predetermined standardis selected from said plurality of transformation bases, the pluralityof transformation bases includes DCT base; and in said first step, atransformation base to be used in said second step is selected based onflag information obtained by decoding said encoded image signalindicating either DCT base or the plurality of transformation bases. 89.The method, as claimed in claim 88, wherein, in the case where the flaginformation indicates the plurality of transformation bases other thanDCT base, one of the plurality of transformation bases other than saidDCT base is selected based on the similarity relationship of saidcharacteristic of brightness distribution of said reference signal.